This article concerns the foam type of bubble bath (suds on top of the water), not the fizz type (bubbles throughout the water). The gush of faucet water falling straight down may be insufficient to produce enough foam, leading to use of more product per bath. Drawbacks to that are increased cost and greater exposure to irritation. The technique given here maximizes foam without jets, hoses or excessive use of product.
Put 2 inches of water in the bathtub, and dissolve the desired amount of foaming preparation in it.
Leaning in from outside the long side of the tub, with fingers spread and palms down and horizontal, use rapid (at least twice a second) opposing (toward and away from each other) long arm motion (from the shoulder) to splash the water very hard. It's like trying to clap very loud, but with thumbs, not palms, facing each other. Continue until the foam stops rising.
Rapidly run in the rest of the water. Either enter the tub while starting to fill it to its final depth, sitting as far as possible from the faucet, or add the rest of the water before entering, pushing the foam away to make space to sit.
If your water is anything but fully "softened" and you want to retain the foam while washing, wash with something other than soap. I mean soap in its technical sense, which includes only some of the many toiletries thought of as “soaps.” The following rule of thumb covers most cases: If it's a solid, it's probably soap unless it's Dove or Caress; if it's a liquid, it's probably not soap, unless it's Dr. Bronner's or an imitator health-food type brand. Dove and Caress don't contain enough soap to rapidly de-foam a bubble bath in unsoftened water. There are other supermarket bars that are similarly "non-soap," but they change formulas from time to time, and Dove and Caress have been close to the same formula for a long time.
If you still have suds when you're done, follow the opposite to the procedures above to make draining easier by destroying the foam. Before getting out, splash the water a little with a real bar of soap (almost any but Dove or Caress) or liquid (Dr. Bronner's or the like), and if your water is fully "softened," "harden" it with a handful of Epsom salt.